1. Field of the Invention
The present invention relates to a semiconductor memory device, more particularly to a random access memory (RAM) with a shift register for high-speed reading and writing. The RAM according to the present invention can be used for a video RAM.
2. Description of the Related Art
High-speed read/write shift registers are used in a video RAM for storing the picture data for a cathode ray tube (CRT) display in order to treat a large amount of data in a short time.
In order to increase the resolution of a picture on a display plane, it is necessary to increase the number of pixels of the picture. This requires the number of memory cells of the video RAM, and accordingly, the number of bit lines and word lines, to be increased. If the number of bit and word lines is increased, the number of elements (bits) of the shift register is increased. However, it is very difficult to carry out shifting-in and shifting-out of the data for a shift register having such a large number of bits.
There is a need for changing the data stored in a video RAM in order to change the picture displayed on the display plane. It is necesssary to read the data from the video RAM and simultaneously to write new data into the video RAM, when displaying a quickly changing picture.